The present invention relates to a fuel cell stack, a fuel cell, an intermediate layer for a fuel cell or fuel cell stack, and a method.
A fuel cell has a sandwich-like structure with an electrolyte disposed between end plates. Between the electrolyte and one end plate there is an anode, and between the electrolyte and the other end plate a cathode. Solid and liquid electrolytes are known; accordingly, the electrolyte can be accommodated in a supporting structure or may itself possess the required solidity to enable it to be built into the cell. The operating temperatures are likewise significantly different, varying from ambient to several hundred degrees C. or more.
A known practice is to combine individual fuel cells to form a fuel cell stack as a means of obtaining the required operating voltage by connecting a suitable number of individual cells in series.
In addition to the individual fuel cells, a fuel cell stack then contains, on each side of the stack, special end plates which complete the stack, and preceding each of said end plates a connection plate (e.g. gold-coated copper plate) with the connections enabling the stack to be connected to the leads of the supplied load.
The abovementioned elements are stacked until the fuel cell stack contains the required number of fuel cells and are then clamped together including the connection and end plates. This can be performed by tie rods which run through the entire stack and are bolted to the end plates.
Assembling a stack of this kind is an intricate process, as all the elements have to be individually placed on top of one another and held in the correct fit until they are bolted together.
In addition, e.g. the connection plates are expensive to manufacture.
Assembly can be simplified by using bipolar plates. A bipolar plate according to the prior art is produced e.g. by implementing the end plates of two adjacent fuel cells as a single piece, with the advantage that, when assembling a plurality of fuel cells to form a fuel cell stack, the number of elements to be assembled can be reduced. Cooling channels can additionally be disposed in the bipolar plates, resulting in considerably improved thermal management of the fuel cell stack.
Current flows in a bipolar plate during operation of a fuel cell stack, as said plate constitutes the electrical connection between the anode of one adjacent fuel cell and the cathode of the other adjacent fuel cell.
It is likewise possible to bond together adjacent end plates of the individual fuel cells to form a bipolar plate.
Instead of clamping, individual elements or all the elements of the fuel cell and of the entire fuel cell stack could also be bonded together, which yields an operational solution. This is likewise an intricate process:
Checking the parts to be bonded for damage such as scratches, etc., evenly applying the glue, drying, drawing off solvent vapor, bonding under pressure and temperature, cleaning off excess adhesive (blockage of gas supply and cooling channels, etc), checking that all the media such as air, hydrogen, water, etc. are properly separated from one another, etc.,—all the foregoing prevent quick and easy assembly of the elements to form a fuel cell stack.
A fuel cell stack may contain a large number of fuel cells: for example, a stack of fuel cells having a polymer membrane as an electrolyte, producing 7 kW and weighing approximately 20 kg possesses around 100 cells.
There also arises the question of testing a newly manufactured fuel cell stack and of repair and maintenance: individual defective cells must be able to be removed from the stack, repaired or replaced and reinserted. Although this is essentially not impossible in the case of a bonded stack, it makes little sense because of the time and effort involved. Even with a clamped stack, the time and effort is considerable: the clamp must be undone, which requires particular care in the handling of the cells which do not need to be replaced. These must not detached from one another and also in particular as a unit, so as not to disturb the original fit of the individual elements.
A disadvantage further arises when using bipolar plates: the selected cell can only be replaced along with the end plates of the adjacent cells. The adjacent cells therefore have to be dismantled; electrodes as well as electrolyte lose their original fit. The advantage for manufacturing the fuel cell stack becomes a disadvantage in the stack's later life.
It has therefore become a known practice to form packages of two end plates and seal them with a kind of O-ring seal, thereby creating a unit that is functionally identical to the bipolar plate. Precise machining of the abutting surfaces of the end plates is critical here, as the contact resistance must remain small (otherwise the fuel cell stack will lose efficiency) and the cooling channels must remain tight. The outline and sealing quality of the ring seal are likewise critical, as the gas supply channels must be sealed off from one another depending of the design of the fuel cell stack (one channel carries e.g. oxygen and the other hydrogen). The ring seal is expensive in itself (because it must be individually manufactured for particular end plates) and mounting it is difficult and therefore costly.
The abovementioned disadvantages also apply to the manufacture and assembly of the connection and end plates of the stack with the associated seals.
For the purpose of simplifying the description, such packages comprising two joined end plates and similar to bipolar plates,—although not implemented as a single piece—will hereinafter likewise be referred to as bipolar plates, as these packages have the same function in the fuel cell stack as the single-piece bipolar plates.
As a result either single-piece bipolar plates can be used, with the disadvantage that when a fuel cell is removed from the fuel cell stack, dismantling of adjacent cells is unavoidable. Alternatively, disassemblable i.e. multi-piece bipolar plates as described above can be used which, however, involve considerable effort and cost in terms of manufacture and joining together.
There has hitherto been no known design which can provide the proper functionality provided e.g. by the single-piece bipolar plate while at the same time simplifying assembly in terms of effort or handling or which will also allow the manufacturing, assembly and maintenance cost/complexity to be reduced for the ends of the fuel cell stack with their connection and end plates which must be kept fluid-tight.